We will study by serial section the conduction system of hearts in which His bundle recordings have been done, in which atrial arrhythmias have existed during life, in which left bundle branch block existed, in which arrhythmias have occurred after heart surgery, including right bundle branch block, and in which congenital heart block has existed. We will study by serial section the conduction system and both AV rims in which WPW had existed. We will continue to examine grossly most of the congenital hearts obtained at autopsy in the City of Chicago, studying new complexes as they are recognized. We will finish a book describing our experiences with more than 3,555 congenital hearts. We will study histologically the valves of various entities to see if the valves are congenitally malformed or hemodynamically altered. We will study the myocardium histologically in various congenital entities using normal hearts of the same age as controls to see if its changes contribute to the cause of death. We will study the natural history of the ductus arteriosus in various complexes to understand delayed closure, and the conditions under which it is propitious for life, or deleterious to life.